The present invention relates to an optical fiber used for light amplification utilizing stimulated emission phenomenon and as a medium for laser oscillation.
Among optical fibers that are generally used as amplifying elements for directly amplifying signal light by utilizing a stimulated emission phenomenon, those of double-core type as shown in FIGS. 2 to 4 are conventionally known.
The double-core type optical fiber 1 is constituted such that a 2nd core 4 and a clad 6 are sequentially formed on an outer circumference of a 1st core 2.
The 1st core 2 is of quartz series with an outer diameter thereof being set to be either of single mode or multi mode, and a rare-earth element (such as Nd or Er) is doped within the 1st core 2.
The 2nd core 4 is also of quartz series, similarly to the 1st core 2, and has a cross-sectional area that is sufficiently larger than a cross-sectional area of the 1st core 2 such that an excitation light from an excitation light source such as a laser diode having a large emission area can be guided into the optical fiber at high efficiency, and is thus of multi mode with respect to the excitation light.
The clad 6 is made of a high molecular resin such as urethane acrylate series or polymethyl methacrylate for purpose of protection as well as confinement of light.
Concrete sizes of the optical fiber 1 may be set such that, for example, an outer diameter of the 1st core 2 is approximately 10 xcexcm, an outer diameter of the 2nd core 4 is approximately 400 xcexcm, and an outer diameter of the clad 6 is approximately 450 xcexcm. As for refractive index distribution, for example, it is set such that a refractive index n2 of the 1st core 2 is approximately 1.463 to 1.467, a refractive index n4 of the 2nd core 4 is approximately 1.45 to 1.46 and a refractive index n6 of the clad 6 is approximately 1.40 as shown in FIG. 3, and is set in a step-wise shape such that the refractive index becomes gradually smaller in getting further outward.
The double-core type optical fiber 1 amplifies the signal light in the following manner. That is, for example, in the case of an optical fiber in which Nd is doped within the 1st core 2, the signal light in a zone of 1.06 xcexcm is made incidence into the 1st core 2 while the excitation light in a zone of 0.80 xcexcm is made incidence not only into the 1st core 2 but also into the 2nd core 4. Pumping of the 1st core 2 is performed by the excitation light propagating through the 1st core 2 and the 2nd core 4 so as to amplify the signal light. Especially, the excitation light propagating through the 2nd core 4 acts to excite the 1st core 2 when crossing the 1st core 2, as shown in FIG. 4.
In this manner, since the double-core type optical fiber 1 can guide the high output excitation light within the 2nd core 4 occupying a comparatively large region around the 1st core 2 even when it is difficult to directly make incidence the high output excitation light into the 1st core 2, so-called lateral excitation effect can be obtained, so that there is an advantage to be capable of efficiently performing the light amplification.
However, in the double-core type optical fiber 1, while the excitation light propagating through the 2nd core 4 acts to excite by being absorbed by the 1st core 2 when crossing the 1st core 2 as shown in FIG. 4, such only lateral excitation is not sufficient to improve excitation efficiency since a length at which the excitation light propagating through the 2nd core 4 intersects with the 1st core 2 is considerably short.
Moreover, among the excitation light made incidence within the 2nd core 4, a light having a component that do not intersect the 1st core 2 merely turn within the 2nd core 4 along a circumferential direction but not contribute to excitation of the 1st core 2.
Due to above-described facts, there are limits in securing sufficient excitation efficiency by using the structure of conventional the double-core type optical fiber.
An object of the present invention is to further improve excitation efficiency than the prior art such that the excitation light propagating through the 2nd core is guided into the 1st core at further higher frequency than the prior art in the double-core type optical fiber.
As disclosed in Japanese Unexamined Patent Publication No. Hei 6-235808 (1994), for example, it is known for a method of applying periodic refractive modulation to a core of an optical fiber so as to form a grating by periodically irradiating an ultraviolet light onto the optical fiber from an external laser light source via a diffraction grating.
The grating formed in such optical fiber, especially a long period grating, is conventionally used as a filter for eliminating the light from the core by coupling the light adjacent to a wavelength corresponding to the period with a clad by means of mode coupling.
However, in the double-core type optical fiber, since a 2nd core is formed on an outer circumference of a 1st core, a formation of a long period grating in the 1st core will cause introduction of light propagating through the 2nd core into the 1st core by mode coupling, contrary to the function of the above-described filter.
The present invention has been made in view of such an aspect, and a long period grating has been formed in the 1st core in so-called double-core type optical fiber. With this structure, the excitation light propagating through the 2nd core will be guided into the 1st core by mode coupling and will be propagated along an axial direction of the 1st core, so that so-called axial directional excitation is performed and higher excitation efficiency when compared to lateral excitation only can be obtained.
Further, in the above-described improved optical fiber of the present invention, the long period grating is respectively formed for respective modes propagating to the 2nd core. With this structure, since the excitation light is mode coupled for each of the modes to guide in the 1st core by using the long period grating corresponding to each of the modes, the excitation efficiency is further improved.
Further, in the above-described improved optical fiber of the present invention, the plurality of long period gratings are provided and a distance between adjacent long period gratings is set to be a distance that is sufficient to enable the rare-earth elements absorb the excitation light propagating through the 1st core. With this structure, the excitation efficiency is further improved.
In the above-described improved optical fiber of the present invention, a clad is further formed on an outer circumference of the 2nd core and the clad is composed of a substance transmitting the ultraviolet light. With this structure, by periodically irradiating the ultraviolet light to the 1st core, it is possible to make the ultraviolet light reach to the 1st core 2 without exfoliating the clad when forming the long period grating.